Characterising the contemporary stress orientations near an active continental rifting zone: A case study from the Moatize Basin, central Mozambique

This paper presents the first comprehensive analysis of the present-day stress from boreholes near an active continental rifting zone in the Moatize Basin, Mozambique. The state of present-day stress in this area that is located ca. 100 km away from the Eastern African Rift System (EARS) is poorly understood, and most of our knowledge is from earthquake focal mechanisms that provide stress information from the deeper part of the lithosphere, and to a lesser extent from surface geological features in the Malawi region. Considering the limited reliability of earthquake-derived stress orientations near plate boundaries, poor coverage of low to moderate magnitude earthquakes in eastern Africa, and ambiguity about the latest activity of geological structure; other well-established methods are required to shed light on the active tectonics of EARS. In this study, we analyse stress orientation using log data from 95 vertical boreholes in a mine site to investigate the neotectonic stress pattern of the region. Analysis of 17.9 km of televiewer logs resulted in interpretation of 1188 stress-related borehole failures. The results indicate a mean regional trend of 045 degrees +/- 31 degrees for the maximum horizontal stress (S-Hmax). Our investigation reveals that the regional state of stress in the study area is controlled by superposition of stress sources that act at very different spatial scales. The consistency between our results and predictions by plate-scale geodynamic models of stress orientations confirm that the regional pattern of stress in this area is mainly controlled by first (>500 km) and second (distances between 500 and 100 km) order stress sources (i.e., large tectonic forces and lateral density variations). However, high-resolution data used in this study reveal that third (between 100 and 1 km) and fourth (<1 km) order stress sources from stiffness contrasts, rock fabric and geological structures have a great impact on the stress perturbations at smaller and local scales.